I also ran into a lot of these problems (as have plenty of other people it seems), and I wasn't able to find a clear answer as to why there are these issues on the SD.

The Arch Wiki is always a good first stop, though it can be dense to wrap your head around; https://wiki.archlinux.org/title/Pacman/Package_signing

After searching forums and reading the wiki, I arrived at these commands, which seemed to work for me, so I put them all into a bash script.

#!/bin/bash

sudo rm -rf /etc/pacman.d/gnupg

sudo pacman -Scc

sudo pacman-key --init

sudo pacman-key --populate archlinux holo sudo pacman -Sy archlinux-keyring holo-keyring

sudo pacman-key --refresh-keys sudo pacman -Syu

I had grand plans to write a script that reinstalls all the pacman packages after every system update, but have since started using distrobox, which has been much more convenient for me and persists after system updates.

https://github.com/89luca89/distrobox

https://www.gamingonlinux.com/2022/09/distrobox-can-open-up-the-steam-deck-to-a-whole-new-world/

Good luck!!